The objective is to provide a potential new therapy for cancer, particularly breast cancer, and conceivably prostate cancer, as based upon the forthcoming chemistry of the prolactin inhibiting hormone, and the synthetic hormone. A preparation of ca. less than 100 ng of the prolactin inhibiting hormone has been achieved through extensive isolation steps. This preparation could be essentially pure, because of symmetrical single peaks by repeated high pressure liquid chromatography in two systems, and an in vitro potency at ca. less than 5 ng. The paucity of ca. less than 100 ng per 80,000 hypothalami necessitates additional processing for more definitive data on purity and structure. The weight was estimated by comparing uv. spectra at 220 nm with that of model synthetic peptides. This preparation is not a catecholamine by chromatography, and gives timely credence to the concept that prolactin is mediated by complex mechanisms, including a peptidic inhibiting hormone and a catecholamine. This preparation is now being subjected to field desorption mass spectrometry. 380,000-480,000 additional fragments are at various stages of processing. Microscale electrophoresis may follow h.p.l.c. Sequencing will be achieved by classical reactions for which we already have experience and/or by field desorption mass spectrometry which we already have in progress. The characteristics of this apparent peptidic PIH indicates a low molecular weight peptide comparable with TRH, LHRH, and SRIF, having 3, 10 and 14 amino acids, respectively.